“Retroreflectivity” refers to the ability of an article, if engaged by a beam of light, to reflect that light substantially back in the direction of the light source. Beaded retroreflective constructions are known in the art and generally include a plurality of transparent spherically shaped beads affixed to a substrate along at least one major surface thereof. Exemplary retroreflective articles include signs, security cards, pavement markings and garments. In beaded retroreflective constructions, substantially collimated light (e.g., a beam of light from an automobile headlight) enters the front surfaces of the beads, is refracted, and impinges on a reflector at or near the back surfaces of the beads. The optical characteristics of the beads and reflectors can be tailored so that a significant amount of light is returned antiparallel or nearly antiparallel to the incident light.
Typical reflectors include those utilizing pigments as well as conformal reflective coatings that have been applied to the back side of a layer of transparent beads, for example in a planar construction comprising a monolayer of beads adhered to a backing or substrate. Pigments can be used as reflectors by first dispersing them in a binder and coating the dispersion onto the back surface of a layer that comprises a plurality of beads or by partially imbedding a layer of beads in the pigmented binder. Reflective pigments include, for example, titania particles, mica flakes, other powders and the like. Conformal reflective coatings are normally applied to the back side of the reflective beads (e.g., between the beads and the substrate) in a planar construction. Conformal reflective coatings include metal thin films such as aluminum and silver, and dielectric coatings such as metal fluorides and zinc sulfide. In the construction of retroreflective articles, choosing one type of reflector over another can depend on a variety of factors including the desired daylight appearance of the article, material and manufacturing costs, the desired level of retroreflective brightness and the overall structure of the article and the associated manufacturing processes. Reflectors made from pigment are normally inexpensive to manufacture and can provide desired daytime features, such as a bright white appearance when exposed to bright, diffuse lighting. Conformal reflective coatings applied to the back side of reflective beads in planar constructions produce high retroreflective brightness but can be expensive, metallic grey in color, or have other less desirable features.
Bead cores coated with a single complete concentric optical interference layer are known to produce retroreflective elements that provide covert interference colors and retrochromic patterns. The term “retrochromic” refers to the ability of an article or a region thereof, when viewed in retroreflective mode, to exhibit a retroreflected color that is different from the color exhibited when the object or region is viewed in diffuse lighting. The art has also noted an effect of the refractive index of a single complete concentric optical interference layer on the saturation and intensity of retrochromic colors. It has been suggested that the medium behind the optical interference layer (e.g., between the retroreflective element and the substrate or backing) can provide a high refractive index contrast interface between the coating and the medium. The art suggests that a thicker coating applied to a bead already comprising a complete concentric optical interference layer, can be used to adjust the interference effect by fixing refractive index differences of the interfaces.
Further improvements are desired in retroreflective elements and in articles comprising such elements.